XOR Command meeting minutes 1/9/95 -- Document number X3T10/95-114r0
Jay_Elrod at notes.seagate.com
Wed Jan 11 10:48:40 PST 1995
XOR Command Study Group meeting minutes - Document number X3T10/95-114r0
Meeting place: Harrah's, Lake Tahoe
Date: January 9, 1995
Gerry Houlder Seagate
Jay Elrod Seagate
Bill Hutchison Hewlett Packard
Doug Prins Q-Logic
Larry Lamers Adaptec
Paul Hodges IBM
Keith Staub Quantum
Paul Peterson Conner
James McGrath Quantum
John Baudrexl Fujitsu/Intellistor
George Penokie IBM
Bob Snively Sun Microsystems
Paul Boulay Hitachi Computer Products
Thai Nguyen StorageTek
Duncan Penman IIX
Giles Frazier IBM Austin
Jeanne Martin LLNL
Lansing Sloan LLNL
Gene Milligan Seagate
Gerry Houlder acted as chairman for the meeting. The issues discussed are
1) Newly added 10 byte XDWRITE command - the NDisc bit will be moved from
bit 5 to bit 2 (of byte 1) to more closely resemble the 16 byte XDWRITE command.
2) RAID Group Address mode page - the concept of multiple RAID groups, LBA
ranges, etc. will be removed. (The whole mode page may be removed for the time
being, until a need is demonstrated for a more-than-one-byte physical secondary
3) Opcodes for XOR commands were presented, with no objection. The six new
80h = XDWRITE (16)
81h = REBUILD
82h = REGENERATE
50h = XDWRITE (10)
51h = XPWRITE
52h = XDREAD
4) The NDisc bit will be put back into the 16 byte XDWRITE command for those who
wish to use the XDWRITE command (rather than REGENERATE) during a controller
supervised rebuild or regenerate operation. (A "controller supervised" rebuild
regenerate is one where the devices do not have peer to peer capability, and the
controller must send XOR commands to all devices involved.) The NDisc bit will
occupy bit 2 of byte 1.
5) Once again, the issue of whether to consolidate the MAX XDWRITE SIZE and
MAX XPWRITE SIZE fields in the RAID Control mode page was discussed. The
two fields will be consolidated into one field which will occupy bytes 4 - 7 of
RAID Control mode page. Bytes 8 - 11 will be reserved.
6) It was recommended that there be information available in the RAID Control
page regarding total buffer space available for XOR type operations. No final
was reached as to how this would best be done. It was agreed, however, that
should be a "buffer full" status byte to indicate when there is no more room
in the buffer. This buffer full condition results (for example) from XOR data
sitting in the
buffer waiting for an XDREAD command. A "buffer full" status byte will be
the appropriate committee.
7) There was discussion regarding how long XOR data should remain in the buffer
of an XOR device following an XOR command which would be expected to be followed
by an XDREAD command (10 byte XDWRITE, for example). This issue will be covered
in the next release of the XOR document, stating the events which can cause the
data to be cleared.
8) It was recommended that an implementor's note be added to any commands which
require a follow up XDREAD command, stating that linking of the XDREAD command
to the associated prior command is recommended for data integrity.
9) It was recommended that the models at the front of the XOR document be
to include more examples of possible RAID configurations, along with recommended
procedures for executing RAID functions in those configurations.
10) For reasons of SPC compatibility it was recommended that the 16 byte XDWRITE
command be restructured. As a result of this discussion, the Secondary Address
will be moved from bytes 12-14 to byte 14 (reducing it from 3 bytes down to 1
the Transfer Length field will be expanded from bytes 10-11 to bytes 10-13.
11) For reasons of compatibility with the REBUILD and REGENERATE commands, the
Secondary Control field of the 16 byte XDWRITE command will be moved to bits
12) It was recommended that an "XOR Disable" bit be added to the RAID Control
page to allow the controller to disable any XOR functionality within the
thinking is that in some designs this may free up any buffer space which may
otherwise been designated for use in XOR operations. Bit 1 of byte 2 of the
mode page will be used for the XOR Disable bit.
13) It was recommended that the Mirror bit be removed from the 16 byte XDWRITE
command since the operation of the command is radically altered by this bit to
that no XOR functions are performed. This bit will be removed in the next
14) It was recommended that an XOR bit be added to Inquiry data, indicating XOR
functionality in the device. This will be proposed to the appropriate committee.
15) It was recommended that the statement on page 22, regarding how far an
command shall have progressed before sending good ending status when write
is enabled, be removed. This will be done.
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